It isn't a matter of RTFM- just about any firearm manufactured for .223 Remington will handle 5.56mm milspec, just like any .308 should handle 7.62 NATO, or any .30-06 will handle, well, .30-06. Even though the manuals might caution against military ammo, and SAAMI might say there is a difference, the manufacturers KNOW that these guns will at some point be fed military surplus fodder. The only time I might say it is critical is something like the ADC Model 1 derringers, in the .223 loading, due to just how little metal there is, but for something like this that started life as a 12 ga shotgun action, there's enough metal there that I think it would more than compensate for the higher pressures.

These little "handiguns" might another exception, but we would have been hearing a lot about this if it was a chronic issue. I'm wondering if there was a flaw in the chamber, or the action might not have been entirely locked. I'd also like to find out if you have a lot number on that ammunition, and run it down, becuase if the case itself failed, then this wouldn't be an isolated incident, odds are something would be bad with the lot.

This wouldn't happen to be cheapo European ammo, would it? I've thrown a lot of US military ammo out of my Contender, and I've never seen anything that even looked like it was bulging, much less ruptured, but I've heard a few horror stories about cases made in the old ComBlock.